[lldb] Creates _liblldb symlink from cmake

Summary:
This is another attempt of D67993.

This change removed hard coded relative paths. This way we can generate correct result when get_python_lib() returns a different path, or LLDB_PYTHON_RELATIVE_PATH is specified directly.

By moving things out of python, we are also able to correctly process more cross compile situations. E.g. .pyd vs .so for Windows.

Subscribers: lldb-commits, mgorny

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68858

llvm-svn: 374953